Poisonous Perfumes

Jeff Mitton

Natural Selections (appeared in the Daily Camera July 7, 2000)

Perhaps you remember, long ago, a television commercial for Grape Nuts Cereal
that started with Euell Gibbons asking earnestly “Did you know you could eat a pine
tree?” He asked the wrong question. What Gibbons should have asked was “Did you
know you can eat pine seeds?” The needles of pine trees are certainly not edible.

Consider the plight of a sedentary, motionless tree, maintaining hundreds of
pounds of needles for several centuries. How does it defend itself against mice, squirrels,
raccoons, bears, deer, elk, and thousands of species of herbivorous insects?

Defensive compounds are the primary deterrent for the majority of natural plant
species. Ponderosa pine, Pinus ponderosa, provides examples of two modes of chemical
defenses.

Ponderosa needles contain several compounds, such as phenols, that are stored in
intracellular compartments. When an animal chews needles, it crushes the compartments,
mixing phenols with the other compounds in the cell. The phenols bind the proteins,
making them insoluble and indigestible. So a mule deer could nibble needles until its
belly was full, but it would get no proteins and virtually no calories from this meal. Most
animals quickly learn the futility of eating pine needles.

While some compounds protect trees by depriving herbivores of nutrition, other
compounds disrupt the metabolism of the herbivore. Western ranchers have known for a
century that consumption of ponderosa pine buds and needles causes abortions in cattle
and sheep. The abortion-causing activity of ponderosa needles has been confirmed in
experiments with laboratory mice, but the agent that causes the abortion is a matter of
controversy. Some studies have purified several resin acids, such as pimaric acid, from
needles and demonstrated that they cause abortions in mice. But other studies have noted
that the needles contain the bacterium (Listeria monocytogenes) that causes the disease
listeria, which results in abortion. It is possible that the resin acids and the bacterium
work together to produce abortions.

The abortion-causing activity of ponderosa pine restricts reproduction of mule
deer in our Boulder Mountain Parks. Dr. Chuck Southwick monitored the deer population
at the National Center for Atmospheric Research in Boulder, CO, for many years,
documenting the size of the herd and the number of young produced. He noted that there
were several winters (1983-1985) during which deep snow lingered for long periods,
blocking access to the grass. When they were unable to eat grasses, the deer browsed on
ponderosa pine. In the springs following these winters, very few live born fawns were
produced.

Natural selection is defined as the differential reproduction of genotypes. Natural
selection via abortion will select against deer and other herbivores that have a
genetically-based propensity to feed on ponderosa pine.

Pine needles have been used to produce abortions in many societies, for a long
time. Dioscorides and Pliny the Elder, physicians writing almost 2,000 years ago, both
reported that consumption of pine needles caused abortion. American Indians, including
the Arapaho, independently acquired and used the same knowledge. During harsh
circumstances, a pregnancy had a low probability of producing a healthy child and it
endangered the life of the mother. The loss of a baby was a sad but not unusual event, but
the loss of a mother was a tragedy that jeopardized the health and safety of young
children. If the family decided to protect the life of the mother by terminating the
pregnancy, a tea of ponderosa needles was used to produce an abortion.

We enjoy the smell of a pine forest during the summer. The bouquet of fragrances
is neither perfume nor nectar, but chemical defenses evolved by trees in response to
herbivores.

 

Ponderosa pine, Pinus ponderosa, in the Guadalupe Mountains

Photo by Jeff Mitton

 

Ponderosa pine, Pinus ponderosa, near Bear Lake, CA

 

Photo by Jeff Mitton